Many low voltage electronic circuits have a need to drive high voltage switching transistors through an isolating barrier. This need stems from the fact that a voltage differential of at least 100 volts typically exists between the low voltage control logic and the high voltage power switch. Therefore, for safety and other reasons, the control logic and high voltage switch must be electrically isolated.
Several circuits addressing these problems have been previously proposed. For example, circuits utilizing a number of low voltage power supplies coupled to the power switches, high-speed opto-couplers, or other extensive electrical isolation circuits have been proposed, although all have met with limited success due to problems associated with their excessive size and cost, low reliability due to the large number of components, or the inability of the circuit to control a power device over the full duty cycle range of 0 to 100%.
Transformer coupling of low voltage control signals to the high voltage power switches provides several advantages such as electrical isolation and step up or step down capabilities. Unfortunately however, transformers are generally only capable of coupling higher frequency AC signals since the transformer quickly becomes saturated by any DC signal components. For this reason, many prior art transformer isolated semiconductor drive circuits have been limited to 50% duty cycle with roughly equal positive and negative volt-seconds to avoid transformer saturation.
Prior art transformer isolated semiconductor drive circuits utilizing a modulated drive signal are generally still limited to a duty cycle ratio of between 1 and 99%, to prevent transformer saturation. Oftentimes, however, a duty cycle of 0% or 100% is required and cannot be provided by known circuits.